The present invention relates to a device including a spreadable dowel for securing a fastener insert, particularly a threaded fastener insert, into a panel, particularly a sandwich type panel having a core, for example a honeycomb core between two cover layers. The present invention also relates to a spreading dowel specifically constructed for use in such a device.
German Patent Publication (DE-OS) 2,061,347 discloses a sandwich type lightweight structural panel with a core between two covering layers into which a dowel is inserted into a bore. The dowel is provided with spreading elements for receiving and anchoring a threaded insert. In this prior art the threaded insert is first introduced into the bore and thereafter a sleeve is inserted into the bore. Thereafter, a threaded mandrel is inserted through the sleeve and threaded into the threaded insert. Thereafter, the threaded mandrel is pulled axially outwardly, whereby the threaded insert is pulled into the sleeve with a frictional contact. As a result of this pulling operation a throat in the sleeve is widened to form a bulging auxiliary head which cooperates with a main head of the sleeve in clamping the upper covering layer of the lightweight structural panel. Additionally, due to the pulling action and due to a spreading effect of the threaded insert, anchoring rods provided in the sleeve are caused to spread and thereby pressed into the core of the panel. A further anchoring is provided by a milled or knurled edge of the threaded insert which bites into the sleeve wall.
It is a substantial disadvantage of this type of prior art method that it is necessary to pull the threaded insert into the sleeve. This operation requires a special tool including a threaded mandrel which must be first threaded into the threaded insert and which after the pulling operation is completed must be threaded out of the threaded insert. Additionally, the pulling operation must be performed by a fluid operated piston cylinder arrangement, the piston of which is secured to one end of the threaded mandrel. Thus, the tool which itself is not of simple construction, depends on a hydraulic power supply.
Furthermore, after completion of the pulling operation a void or hollow space results below the threaded insert whereby the sandwich panel is weakened at that point. Thus, it has been found that the sandwich panels that may be used according to the prior art cannot be of too light a structure. For example, the core may not be of the so-called honeycomb type because the anchoring as disclosed in this prior art cannot be accomplished in a satisfactory manner in a honeycomb type panel with the aid of the prior art dowel in the form of a sleeve. Thus, the prior art method is primarily limited to a compound layer panel having a core of intermediate density such as a synthetic foam material core.